The projection of motion picture film generally requires a device that can transport individual film frames from a film supply system to a projection aperture at some defined rate. Typically, the standard rate is 24 frames per second. The individual frames, while supplied at a constant rate from the film supply platter, must be decelerated near the vicinity of the projection aperture and held stationary while light is passed through the film and projected onto the screen.
Various devices and methods are known for moving film intermittently for registration at the projection aperture. For example, the Geneva mechanism moves film one frame at a time into the projection aperture by the application of an intermittent force applied on the perforations of the film.
Other examples of methods and systems of advancing film use electronic means to intermittently drive a film advance sprocket, such as described in U.S. Pat. Nos. 3,819,258, 4,022,525, 4,697,896, 5,011,201, and 5,875,020. For example, U.S. Pat. No. 3,819,258 describes an improvement to a motion picture system in which an intermittent sprocket for advancing the film is driven by a servo-motor controlled by signals from optical encoders. The advance cycle is divided into two phases; one for rapidly advancing the film frame into an approximate final position, and a second fine positioning phase where the servo motor positions the frame precisely at the projection aperture.
U.S. Pat. No. 4,022,525 describes another film advance method and system using an electronically driven sprocket wheel in which two different driving systems are used to drive a servo-motor attached to the film advance sprocket. The first system is a velocity controlled servo system in which the servo-motor is controlled by a predetermined velocity profile. When the sprocket advances the film frame to the projection aperture, the servo stops and control circuitry switches to a position controlled servo system for preventing movement of the film in the projection aperture.
U.S. Pat. No. 4,697,896 describes another film advance method and system using an electronically driven sprocket wheel in which there is a period of rapid acceleration of the sprocket, followed by rapid deceleration, and finally by a fine-position adjusting phase. The driving circuit includes using sprocket position sensing to give position and timing feedback for the circuit to refine the amount of current applied in subsequent drive cycles.
U.S. Pat. No. 5,110,201 describes yet another film advance method and system of using an electronically driven sprocket in which the transport of a film frame takes place over three distinct phases: a brief acceleration phase, a longer and gentler deceleration phase, and a final position adjusting phase. Maintaining the sprocket teeth against the leading edge of the perforations reduces stressful impact on the film during the acceleration phase. By providing a longer, smoother deceleration phase the method achieves the desired goals of reducing objectionable noise and excessive wear on the film.
U.S. Pat. No. 5,875,020 describes a motion picture system in which two intermittently driven sprockets, one on each side of the projection aperture, are driven synchronously to advance film frames into the projection aperture. The use of friction or registration pins at the projection aperture to decelerate the film is avoided in this system, because the two drive sprockets control the deceleration and final positioning of the film frame at the projection aperture.
A disadvantage of conventional motion picture projection systems featuring intermittent film advance sprockets can be that they function as both a means to advance film into the projection aperture and as a means of registering the film at the aperture. The latter function requires the intermittent sprocket to come to a complete stop every film advance cycle to prevent movement of the film frame during projection. The end of the film frame exposure period is the starting point for the next cycle, thus requiring the sprocket to rapidly accelerate, and subsequently rapidly decelerate so that there is time to move the film the required distance towards the projection aperture. The intermittent rapid acceleration and deceleration of the film may cause considerable wear at the perforations. In the case of large format film, such as 70 mm film with a frame width of 15 perforations, the wear can be excessive and lead to the rapid destruction of the film.
Another disadvantage of conventional electronically driven intermittent sprockets can be their variability in precision over time. This may be compounded by potential mechanical dimensional errors in the sprocket assembly, bearing play, and the variability in perforation pitch of film prints. These types of systems can require elaborate position measuring systems and/or timing during the frame advance cycle for fine-tuning of the position of the film within the film gate. This latter solution can subtract time from the acceleration/deceleration phase, thereby creating the need for yet higher accelerations in the shortened time period.
One system and method of intermittent film transportation that can reliably move large format film is the rolling loop system and method as described, for example, in U.S. Pat. No. 3,494,524. Generally, with rolling loop systems film frames are advanced by intermittently creating film loops well in advance of the projection aperture and then smoothly rolling the loops towards the gate. Despite the fact that a rolling loop mechanism is relatively gentle in transporting film frames, it has been found that there is a need for supplemental deceleration of the film before it is registered on fixed pins near the projection aperture.
For example, U.S. Pat. Nos. 3,600,073 and 5,050,985 discuss supplemental deceleration mechanisms that, when combined with a rolling loop film transport device, provide an effective and reliable method of transporting and projecting large film formats, including the 70 mm 15 perforation format. Another rolling loop film movement, described, for example, in U.S. Pat. No. 5,587,750, utilizes a smaller rotor to advance the film loops.
All the rolling loop systems mentioned above achieve film advance timing by mechanical means. An important feature of the rolling loop film movements is the fact that the projected images can be an order of magnitude steadier in the projection aperture than they would be using conventional intermittent sprocket and Geneva movements. Rolling loop projection systems can also project images with much greater illumination efficiency than the Geneva and electronic intermittent sprocket film advance movements.
Some conventional rolling loop projection systems advance rolling film loops by means of a rotating circular rotor with film loop gaps. Other rolling loop projection system systems advance film loops linearly. For example, one system is described in U.S. Pat. Nos. 5,341,182 and 5,633,696 and another is described in U.S. Pat. Nos. 6,120,151 and 6,257,752. These linear film loop-advancing systems do not use a circular rotor.
There are several disadvantages with the conventional rolling loop film movements mentioned above. The deceleration cam, while reliable, is expensive to fabricate, to assemble, to set up in the projection system, and to maintain. In addition, film that tails out through the rolling loop movement can cause the deceleration cam timing to change. Should this occur, a trained service technician may need to be dispatched to the theatre to re-align the deceleration cam timing. Partly because of the relative complexity of the cam deceleration device and presence of fixed registration pins, these conventional systems can require specially trained staff for loading and operating the projection system, which adds to the overall expense of operating the motion picture theatre.
There also may be capability limitations of the mechanical rolling loop film movements that make such features as auto-loading film and rewinding film through the projection system head very difficult or not practical to implement. This is primarily because of the way the fixed registration pins and the deceleration cam pins are typically integrated into the projection system film movement system.
Another limitation of these conventional film loop projection systems caused by the fixed mechanical timing relationship between the projection system film input sprocket and the deceleration cam pins precludes the capability of quickly and easily converting the projection system to advance alternate film formats such as 70 mm 8 perforations or 10 perforations, in the case of a 70 mm rolling loop projection system.
In addition to having decelerating cam pins another typical feature of the circular rolling loop projection system may be an air jet system that intermittently applies properly positioned blast of air at the right time laterally to the film at the point where the film loops first begin to form. The air pressure acts to prevent longitudinal bending of the filmstrip that would otherwise interfere with the proper creation of lateral bends of the film loops. A conventional air jet system may include an air compressor, a distribution manifold and a network of copper air tubes attached to the rotor for supplying air pressure at each of the rotor gaps where film loops are initiated. The air jet system may require a significant amount of compressed air that adds to the system cost in terms of space required, the additional equipment required, and the need for regular maintenance. These larger air compressors tend to be very noisy, hence, require an additional sound proofing enclosure.